1. Field of the Invention
The present invention relates to a radio frequency amplification apparatus, usable for wireless communication or the like, including a plurality of radio frequency amplifiers connected to each other.
2. Description of the Background Art
A radio frequency amplification apparatus is required to provide low distortion for keeping linearity and to have high efficiency for reducing power consumption. Especially, a radio frequency amplification apparatus used for digital cellular phones is strongly desired to provide lower distortion and higher efficiency because such characteristics directly contribute to the size reduction of the cellular phones. However, decrease in distortion and increase in efficiency contradict each other. Therefore, a radio frequency amplification apparatus is generally designed to fulfill the standards of the distortion characteristic in accordance with the modulation system of the corresponding cellar phone while maximizing the efficiency.
For solving the contradiction, a distortion compensation circuit technology is conventionally proposed.
As one example of the distortion compensation circuit technology, Japanese Laid-Open Patent Publication No. 2001-144550 (patent document 1) discloses a method of adding a pre-distortion circuit on the input side of the radio frequency amplification apparatus including a plurality of radio frequency amplifiers connected to each other (not shown). Japanese Laid-Open Patent Publication No. 2003-309435 (patent document 2) discloses a radio frequency amplification apparatus including a plurality of radio frequency amplifiers connected to each other as follows. An intermodulation distortion generated by a first-stage radio frequency amplifier and an intermodulation distortion generated by a final-stage radio frequency amplifier counteract each other, as a result of which the distortion of the output is suppressed.
FIG. 8 shows an exemplary structure of a conventional radio frequency amplification apparatus described in patent document 2. The conventional radio frequency amplification apparatus includes an input matching circuit 120, a first-stage amplifier 130, an inter-stage matching circuit 140, a final-stage amplifier 150, and an output matching circuit 160. With this conventional radio frequency amplification apparatus, the size and the bias condition of the first-stage amplifier 130, the circuit constant of the inter-stage matching circuit 140 and the like are adjusted such that the intermodulation distortion generated by the final-stage amplifier 150 is invert in phase to the intermodulation distortion generated by the first-stage amplifier 130 and amplified by the final-stage amplifier 150. As a result, the distortion of the output is suppressed. In this manner, the conventional radio frequency amplification apparatus improves the linearity of the amplification characteristic between an input terminal 210 and an output terminal 220.
However, the conventional radio frequency amplification apparatuses have the following problems. The pre-distortion circuit described in patent document 1 indispensably has an excessively large circuit scale as well known in the art. Therefore, a radio frequency amplification apparatus using such a pre-distortion circuit is not practically usable to cellular phones or the like desired to be more compact.
The conventional radio frequency amplification apparatus described in patent document 2 is not suitable either to devices desired to be compact because the inter-stage matching circuit 140 inevitably has a large scale for adjusting the phase of the intermodulation distortion. In addition, with only the inter-stage matching circuit 140, it is technologically impossible to make an adjustment such that an intermodulation distortion generated by the final-stage amplifier 150 and an intermodulation distortion generated by the first-stage amplifier 120 and amplified by the final-stage amplifier 150 are completely invert in phase to each other, however large the scale of the inter-stage matching circuit 140 may be.